How to Include a common category in a custom Category Search

I don’t think you can express logic like this in query arguments and custom SQL will probably be nightmarish.

I would try to flip the logic. Declare category__in to be list of all categories, but the ones you don’t want.

You can build such list by code, like:

'category__in' => get_categories( array( 'fields' => 'ids', 'exclude' => '7,22' ) ),

PS I am not sure about performance aspect of such query, depends on number of categories involved I guess. Test before using in production.

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